1. Field of the Invention
This invention relates to a photovoltaic cell having a heterojunction of an amorphous semiconductor and amorphous silicon.
2. Description of the Prior Art
Amorphous silicon is obtained by the plasma decomposition of silane (SiH.sub.4). W. E. Spear et al. discovered in 1976 that the conductivity of amorphous silicon could be notably altered by substitutional doping with PH.sub.3 or B.sub.2 H.sub.6. D. E. Carlson et al. manufactured a solar cell using amorphous silicon in 1976. These achievements have drawn attention to amorphous silicon and have given an impetus to researches devoted to improvement of the conversion efficiency of thin-film solar cells using amorphous silicon.
The researches have so far led to development of thin-film amorphous silicon photovoltaic cells of the Schottky barrier type, pin type, MIS type and heterojunction type. The first three of these types promise to provide highly efficient solar cells. The Schottky barrier type photovoltaic cell made by D. E. Carlson et al. in 1977 showed a conversion efficiency of 5.5%, the MIS type photovoltaic cell made by J. I. B. Wilson et al. in 1978 a conversion efficiency of 4.8% and the pin type photovoltaic cell made by Yoshihiro Hamakawa in 1978 a conversion efficiency of 4.5% respectively.
In the case of the pin junction type solar cell, the p or n type amorphous silicon has a short carrier lifetime and, therefore, fails to provide an effective carrier and the p layer which has non-radiative recombination centers suffers from heavy absorption loss of light because it has a higher light absorption coefficient than the i layer.
The inverted pin type photovoltaic cell has been proposed with a view to overcoming these drawbacks. This cell is constructed so that the light impinges on the n type amorphous silicon side. Since this cell has a smaller light absorption coefficient than the p type, it is believed to be more advantageous, though slightly. Nevertheless this n type amorphous silicon is no better than the p type in the sense that it similarly suffers from absorption loss of light.
The inventors made a diligent study devoted to improvement of the conversion efficiency of the pin type photovoltaic cell. They consequency have found that the short circuit current and the open circuit voltage of the pin junction type photovoltaic cell are greatly improved by using in either the p or n layer of the cell exposed to incident light an amorphous semiconductor having an optical band gap of not less than about 1.85 eV, an electric conductivity of not less than about 10.sup.-8 (.OMEGA..cm).sup.-1 at 20.degree. C. and a diffusion potential, Vd, (exhibited in the pin junction) of not less than about 1.1 volts.
The photovoltaic cell of this invention is useful as a photo-electromotive cell in solar cells, photoswitches and the like. Now, the invention will be described in detail below.